Atticus has already made it clear that he isn't afraid of Bob Ewell and thinks his threats are are just talk. As has already been mentioned, Jem is growing up, and he feels the need to prove it. The incident of him showing his chest is just one example of Jem trying to prove that he's becoming a man. Jem's respect for Atticus has also grown, and he feels if Atticus doesn't see Ewell's first threat as something to worry about, then there's no need to bother him with this second threat after Tom Robinson's death -- "only two more to go". Jem makes the "adult" decision not to bother Atticus with this second threat. It makes him feel more manly to make that decision himself and to show Scout that he's mature enough to decide something this important.